1. Field of the Invention
The invention relates to a bearing unit, and in particular to a bearing unit in which a wheel fitting flange is formed on an outer periphery of an inner ring member.
2. Description of Related Art
Conventionally, various kinds of bearing units for supporting a wheel of a vehicle with respect to a suspension in a vehicle body so that the wheel is rotatable have been proposed. For example, as shown in FIG. 3, such a bearing unit includes an outer ring member 2, an inner ring member 3 and balls 4. A flange portion 1, which is fitted to a vehicle body side, is formed on an outer periphery of the outer ring member 2. The inner ring member 3 is rotatably inserted in the outer ring member 2. The balls 4 are rolling elements in double rows, which are reliably arranged between the outer ring member 2 and the inner ring member 3. The bearing unit shown in FIG. 3 is a bearing unit for an axle on a drive wheel side of a vehicle. The inner ring member 3 includes a hub spindle 5 and an inner ring component member 6 that is a member separate from the hub spindle 5. The inner ring component member 6 is fitted to an outer periphery of a vehicle inner side end portion (left side end portion in FIG. 3) of the hub spindle 5. A wheel fitting flange 7 for fitting a wheel to the bearing unit is formed on an outer periphery of a vehicle outer side end portion of the hub spindle 5. Bolt holes 8 are formed at predetermined intervals in a peripheral edge portion of the wheel fitting flange 7. A wheel 10 and a brake rotor 11, which are vehicle side component parts, are fitted to fitting bolts 9 that are press-fitted in the bolt holes 8. The wheel 10 and the brake rotor 11 are fastened together by nuts 13.
The brake rotor 11 is combined with a brake system that brakes the rotation of the wheel 10 so as to decelerate the running vehicle. The brake system brakes the rotation of the wheel 10 by pressing brake shoes (not shown), which are secured to the vehicle body side, to the brake rotor 11 so that frictional force is generated. In this case, in order to uniformly generate the frictional force during braking so as to allow the brake system to appropriately exhibit its performance, it is necessary to make clearances between the brake rotor 11 and the brake shoes uniform. For example, if the clearances are not uniform, the frictional force varies during one revolution of the brake rotor 11, and such variation is transmitted in the form of vibration, to the wheel 10 and the vehicle body. As a result, the phenomenon called brake judder occurs.
The braking judder not only causes the occupants to feel discomfort, but also lowers the braking performance and adversely affects the drivability of the vehicle. Thus, in order to suppress the braking judder, it has been demanded to improve the runout accuracy of the brake rotor 11 after the brake rotor 11 is installed in the vehicle.
As a measure for improving the runout accuracy of the brake rotor 11, it may be considered to improve the accuracy of the brake rotor 11 itself. In addition to the above-mentioned measure, there have been measures that are focused on an improvement in flatness of the wheel fitting flange 7 having a surface to which the brake rotor 11 is fitted.
The fitting bolts 9 are fitted in the bolt holes 8 by interference fit, and an interference is set to approximately 0.1 to 0.5 mm (the interference varies depending upon specifications of the bearing unit). Thus, press-fitting of the fitting bolts 9 into the bolt holes 8 may deteriorate the degree of flatness of the surface of the wheel fitting flange 7. Specifically, by press-fitting the fitting bolts 9 in the bolt holes 8, a bulging portion 12 may be formed on a surface 7a of the wheel fitting flange 7 on a side opposite to a surface on which heads 9a of the fitting bolts 9 abut, as exaggeratedly shown in FIG. 4. A bulging height of the bulging portion 12 may reach a value in a range from 25 to 30 μm. As a result, when the brake rotor 11 is fitted, a fitted surface of the brake rotor 11 abuts on the bulging portion 12, and accordingly, the runout accuracy of the brake rotor 11 is lowered. Moreover, as exaggeratedly shown by a long dashed double-short dashed line in FIG. 4, an outer peripheral edge of the wheel fitting flange 7 may be deformed (warped) toward the above-mentioned fitted surface (toward the wheel). In the case where such deformation occurs in the wheel fitting flange 7 as well, since the fitted surface of the brake rotor 11 abuts on a deformed portion 14 that is deformed as described above, when the brake rotor 11 is fitted, the runout accuracy of the brake rotor 11 is lowered.
In order to suppress the lowering of the runout accuracy of the brake rotor 11 caused by the bulging portion 12 and the deformed portion 14 as described above, it has been proposed to remove the bulging portion 12 and the deformed portion 14 by performing a turning process. However, the turning process is laborious, and accordingly, the cost for the tuning process is caused. As a result, the cost for producing the bearing unit is increased. Further, a problem regarding the quality of the bearing unit may be caused due to adhesion of chips or the like.
Thus, a method for preventing deformation of the wheel fitting flange 7 without the necessity of machining the wheel fitting flange 7 after the fitting bolts 9 are press-fitted has been proposed (refer to, for example, Japanese Patent Application Publication 2007-232169 (JP 2007-232169 A)). In a bearing unit described in JP 2007-232169 A, recesses are formed by removing material at positions around bolt holes to a predetermined depth, in the wheel fitting flange. Accordingly, when the fitting bolts are press-fitted in the bolt holes, regions around the bolt holes are elastically deformed toward the recesses. Thus, it is described that the deformation of the wheel fitting flange can be prevented.
However, in the method described in JP 2007-232169 A, as shown in FIG. 1 to FIG. 3 in JP 2007-232169, it is necessary to subject the portions around the bolt holes to a fine cutting process that cannot be easily performed, and further, the cost for the cutting process is high. Moreover, in the method described in JP 2007-232169 A, since the recesses are formed, the strength of the wheel fitting flange is lowered.